1. Field of the Invention
The present invention relates to an improved diesel fuel which has reduced particulate matter emission characteristics and which contains an effective amount of a dialkyl ether and/or trialkyl ether derivative of glycerol and most preferably the glycerol di-t-butyl ether product mixture prepared by glycerol etherification with isobutylene or t-butanol.
2. Description of the Prior Art
Diesel fuels are known which contain a synergistic cetane improving additive combination of a peroxidic component and an aliphatic polyether of the formula R(--O--X).sub.n O--R.sup.1 where R and R.sup.1 are alkyl groups, X is an alkylene group and n is an integer. See U.S. Pat. No. 2,655,440 and divisional U.S. Pat. No. 2,763,537.
European Application 80-100827.7 describes the use of various propylene glycol mono- an di-ethers as a component of diesel fuels. The compositions described in this reference involve a multicomponent formulation which includes poly-ethers, acetals, lower alkanols, water and only up to 85 volume % diesel fuel hydrocarbons.
U.K. 1,246,853 describes the addition of dialkyl ethers of propylene glycol as smoke suppressants in diesel fuel.
U.S. Pat. No. 4,753,661 describes a fuel such as diesel fuel containing a conditioner which comprises a polar oxygenated hydrocarbon, a compatibilizing agent which is an alcohol, aromatics, and a hydrophilic separant which may be a glycol monoether.
Japanese Published Application 59-232176 describes the use of the di-ethers of various polyoxyalkalene compounds as diesel fuel additives.
The addition of glycol ethers and metallic smoke suppressants have been found to reduce the smoke and soot emissions. These metallic smoke suppressants are typically metal salts of alkanoic acids. Both the health and environmental risks of these salts, especially those of barium, are of concern. See U.S. Pat. Nos. 3,594,138, 3,594,140, 3,615,292 and 3,577,228.
European Application 82-109,266.5 describes the use of ethers to reduce soot. However, a number of these ethers are unable to be used commercially in the U.S. because the resulting fuel does not meet the flash point specification of 126.degree. F. This application also teaches that glycol ethers are not highly effective at reducing exhaust emissions. Based on these teachings, our invention would be unexpected.
Japanese Patent Application 59-232176 teaches that glycol ethers of the formula R.sub.1 --O--(CHR.sub.2 --CH.sub.2 --O--).sub.n R.sub.3 where n is less than five have the effect of reducing particulate, CO and HC emissions which effect is weak. This is in direct contrast to our invention.
Winsor and Bennethum (SAE 912325) describe the use of the ether diglyme to reduce particulate emissions. In addition to being costly to produce, diglyme is highly toxic and has been associated with increased rates of miscarriages. Glycol ethers based on the higher alkylene oxides, especially propylene and the butylenes, are far less toxic than those based on ethylene oxide. Glycol ethers based on ethylene oxide also have unfavorable water partition coefficients. The water partition coefficient for diglyme is greater than 17. Thus virtually eliminating it for any commercial use as a diesel fuel additive.
The addition of dialkyl carbonates and dialkyl dicarbonates, particularly dimethyl carbonate, to diesel fuel has been described to reduce exhaust emissions from compression ignition engines. See U.S. Pat. Nos. 2,311,386, 4,891,049, 5,004,480 and 4,904,279. The high volatility of the lower alkyl carbonates prevents their addition in substantial amounts to typical D-2 diesel fuel. While some dicarbonates have lower volatilities, their poor hydrolytic stability precludes their commercial use.
The Clean Air Act Amendments of 1990 have established certain emission standards for heavy duty diesel engines, in particular with regard to nitrogen oxide and particulate matter emissions. The contribution of diesel fuel sulfur content to exhaust particulates has been well established, and has led to an EPA regulation which will require highway diesel fuels to contain no more than 0.05 wt.% sulfur. In 1991, particulate matter emissions were required to drop from 0.60 to 0.25 grams/BHP-hr., and in 1994 the emission limit is 0.10. Similarly, nitrogen oxide will decrease from 6.0 to 5.0 in 1994 and from 5.0 to 4.0 grams/BHP-hr. in 1998. The California Air Resources Board (CARB) has issued regulations that are viewed as more difficult to meet than the EPA targets. To qualify a diesel fuel in California, emissions must be no greater than the CARB reference fuel which contains 0.05 wt.% maximum sulfur, 10% maximum aromatics and a minimum cetane number of 48.
Many strategies are being used by the industry to reduce emissions. Improved heavy duty diesel engine designs including higher injection pressures, turbocharging, air intercooling, retarded injection timing through electronic tuning control, exhaust gas recycle and exhaust aftertreatment devices all lower emissions.
For this advanced technology to work, a high quality, low emissions diesel fuel is required in addition to the use of various fuel additive improvements including cetane improver use, diesel fuel detergents to keep fuel injectors clean and improved low ash engine oils. A combination of these strategies will be utilized to meet new clean air standards. The key issue is to find the most effective combination of technologies which offer the best cost/performance.
Fuel regulations, especially those promulgated in California, will require costly changes in diesel fuel composition. Desulfurization to achieve the 0.05 wt.% sulfur target is easily accomplished through mild hydrogenation. However, refiners must use deep hydrogenation to decrease aromatic content from the current 20-50% aromatic level down to 10%. Several refiners have elected to exit the California diesel fuel market rather than making the high capital investment required for deep hydrogenation. At least one refiner was able was able to qualify a diesel fuel for California by lowering the aromatics to 19% and increasing the cetane number from 43 for a typical fuel up to around 60 using an alkyl nitrate cetane improver.
The present invention relates to the use of ether derivatives of glycerol which, when incorporated in standard 30-40% aromatic containing diesel fuel, provides reduced emissions of particulate matter, hydrocarbons, carbon monoxide and unregulated aldehyde emissions. For 1994, the engine manufacturer strategy to reduce emissions in order to meet the emission regulations involves using electronic tuning to reduce particulates. In this strategy, nitrogen oxide, hydrocarbons, and carbon monoxide emissions are within EPA requirements. However, for 1998, nitrogen oxide emissions need to be further reduced. If an oxygenated fuel can lower particulate matter emissions another 10-20%, this will provide additional tuning flexibility for nitrogen oxide. The strategy would be to lower particulates to meet the 0.1 gram/BHP-hr. target using a combination of oxygenate additive, cetane additive, and tuning. This widens the window for nitrogen oxide tuning which needs to be reduced from 5.0 to 4.0 grams/BHP-hr. Particulate reductions will also provide an opportunity to further lower nitrogen oxide using exhaust gas recycle. At high particulate matter levels, the particulates block and foul the exhaust gas recycle lines and orifices, and contaminate engine oil. Lower particulates via the use of ether derivatives of glycerol could allow greater use of this new technology.